Mac: Don't repaint scrollbars every frame

cc/output/ca_layer_overlay.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/ca_layer_overlay.h"
 
 #include "cc/quads/io_surface_draw_quad.h"
 #include "cc/quads/solid_color_draw_quad.h"
 #include "cc/quads/stream_video_draw_quad.h"
 #include "cc/quads/texture_draw_quad.h"
@@ skipping 48 matching lines @@
   unsigned resource_id = quad->resource_id();
   if (!resource_provider->IsOverlayCandidate(resource_id))
     return false;
   // TODO(ccameron): Merge the y flip into the layer transform.
   if (quad->y_flipped)
     return false;
   ca_layer_overlay->contents_resource_id = resource_id;
   ca_layer_overlay->contents_rect =
       BoundingRect(quad->uv_top_left, quad->uv_bottom_right);
   ca_layer_overlay->background_color = quad->background_color;
+  for (int i = 1; i < 4; ++i) {
+    if (quad->vertex_opacity[i] != quad->vertex_opacity[0])
+      return false;
+  }
+  ca_layer_overlay->opacity *= quad->vertex_opacity[0];
   return true;
 }
 
 bool FromTileQuad(ResourceProvider* resource_provider,
                   const TileDrawQuad* quad,
                   CALayerOverlay* ca_layer_overlay) {
   unsigned resource_id = quad->resource_id();
   if (!resource_provider->IsOverlayCandidate(resource_id))
     return false;
   ca_layer_overlay->contents_resource_id = resource_id;
@@ skipping 99 matching lines @@
       return false;
     }
     if (skip)
       continue;
     ca_layer_overlays->push_back(ca_layer_overlay);
   }
   return true;
 }
 
 }  // namespace cc